An image sensor has been provided with an effective pixel region for photoelectric conversion, and optical black regions provided in horizontal and vertical end portions of the effective pixel region. The optical black regions are for defining an optical black level.
An image signal formed by photoelectric conversion in the effective pixel region is clamped with the black level obtained concurrently from the optical black regions. Thus, the image signal is adjusted to proper brightness.
As for the adjustment method, there has been considered a method which deals with individual difference in optical black level (hereinafter referred to as difference in OB level) among image sensors by a correction table set in each image sensor before factory shipment, for example, as in a technique disclosed in JP-A-2010-068056.
However, the difference in OB level is not generated only by individual difference. For example, the OB level may change in accordance with heat generated during the operation of an apparatus itself. The difference in OB level that may be caused by the heat cannot be adjusted satisfactorily by the correction table set as mentioned above.